1. Field of the Invention
The present invention generally relates to a system and method for controlling an idle air control valve. Recreational vehicles have different performance and cost requirements from typical automotive applications. This can create unique problems for recreational vehicles particularly during engine start and engine shutdown.
2. Description of Related Art
During engine shutdown NVH (engine shake, piston bounce back) can be a problem. With recreational vehicles this can be quite noticeable given the very close proximity of the engine relative to the driver/rider and the reduced compliance within the engine mount structure. Engine mounts are minimal or non-existent in recreational vehicles to minimize engine roll issues during tip-in and tip-out and because the engine is often part of the structural frame to reduce weight.
Engine starts are less reliable with recreational vehicles because they have fewer cylinders. Fewer cylinders requires a greater rotation before having a cylinder in the proper position to provide power to assist start the engine. In addition, the delay between each subsequent combustion event is longer, thereby making the first combustion event even more critical to engine start.
Many recreational vehicles require a kick or pull start by the operator, which can be highly variable and directly impacts customer satisfaction if the engine is difficult to start. For other systems with electric start, the battery is typically very small and often in a discharged state or poor condition because of the intermittent operational usage of recreational vehicles. ATV's are often used on the snow where the colder temperature also reduces battery performance. These factors give rise to a much higher probability of poor engine starts.
Recreational vehicles tend to have very small plenums such that when the engine commences to rotate during crank the engine creates a higher vacuum in the intake manifold/plenum. The higher vacuum increases the pumping losses so that for a given starting torque the engine will be slower to accelerate and not reach as high a cranking speed over a given time or angular rotation, thereby impacting start performance. Also a higher load impacts the starting feel and effort required for manual pull or kick starts. A conventional automotive application has a large plenum so the pressure is higher which results in better filling of the cylinder during crank, providing higher combustion pressures, higher torque and better engine starts. Conversely recreational applications have reduced cylinder filling during crank resulting in lower cylinder pressure, lower starting torque, and poorer starts in comparison. Both ATV and snowmobiles are required to start reliably and quickly in very cold environments. Unlike their automotive car counterparts they do not have engine block heaters for colder weather operation and are typically used well away from even basic facilities.
Many of the engine start and shut down problems can be addressed by manipulating the idle air control (IAC) valve. Unlike their automotive counterparts, stepper motor IAC valves on recreational vehicles do not have return spring functionality, because the friction from the lead screw that is required to reduce stepper motor torque requirements will not allow the motor to freewheel. This means that prior to starting the engine the stepper motor needs to find a new reference position or use a good last known valve. Finding a new reference during engine start adds a delay to the initialization process. System reliability may be reduced due to the need to drive the motor until the valve hits a hard stop.
To reposition a stepper motor IAC valve to a known position typically requires a strategy of purposely moving the motor against the physical stop at least several steps past the expected hard stop position based on referencing from the last known position. The reason for this is that stepper motors invariably miss steps that require the controller to compensate. Unfortunately, if the number of missing steps is unknown then the amount of compensation required must allow for the worst case. Accordingly, many more steps are required against the hard stop adding to reduced reliability. Additional steps are required since IAC valves rarely have the benefit of a feedback position sensor, for cost reasons, and must operate in open loop mode counting steps. Unfortunately this results in two potential issues; (1) increased valve and gear loading by motoring into a hard stop potentially reducing reliability, depending on the magnitude of hard stepping applied and (2) NVH is created while stepping against the hard stop since the IAC valve is located near the rider on recreational vehicles without the benefit of separation by distance and firewall that occurs with typical automotive vehicles. Also this mode would occur during engine off so there will be no masking effect from engine noise.
One proposed solution includes storing the last known position of the IAC valve during engine shut down. However, storing the IAC valve position for use during the subsequent power-up also has a few drawbacks. One problem with using the last known value is that stepper motors invariably miss steps during operation, therefore, long term maintenance of position through step counting can be unreliable. Storing the last known value requires that the power be sustained within the PCM, increasing system cost. Cost and complexity of the system is further increased by requiring non-volatile memory and a strategy to store the last known good value.
Under certain modes, such as idle speed control, there is less need for accurately knowing the actual valve position since the PID feedback loop will, given time, correct for errors. However, in start up mode any valve position inaccuracy will deteriorate system performance of any feedforward logic (e.g. step change in load such as changing from neutral to in gear with clutch engaged, or A/C on conventional vehicles) since these rely on adding or removing a given quantity of air mass to pre-empt the step change and subsequent impact on engine speed. However, the relationship between the number of steps and airflow rate is not linear, therefore, adding an offset based on the perceived number of steps may result in less accuracy. Similarly, adding or removing the required air mass for the given disturbance, may negatively affect performance.
Other modes such as dashpot mode operate entirely using an open loop, where any error in the IAC position will significantly impact performance. For example, if the actual IAC position is greater than expected based on the perceived number of steps then engine run-on can be an issue, as well as, making parking maneuvers more difficult. If actual IAC position is less than expected then there will be an increase in transmission NVH and difficulties fuelling the small air mass leading to potential misfire, reduced performance, and increased hydrocarbon emissions.
Alternatively some other solutions require the controller to measure the time to nominal current for both normal and waste spark to determine CID. For this to occur robustly there needs to be a significant difference in the cylinder pressure during exhaust and compression stroke. For conventional automotive engines with large plenums this is less of a problem but for recreational applications the plenum volume is often so small that cylinder filling is reduced during crank making CID detection less robust.
In view of the above, it is apparent that there exists a need for an improved system and method for controlling an idle air control valve.